This invention relates generally to the field of electric motor driving systems, and more particularly to a method and apparatus for detecting the rotor position of a mass storage device spindle motor using a mutual inductance zero crossing.
Electric motors include a rotor and a stator having a plurality of wound field coils. Brushless DC motors are electronically commutated, wherein solid-state switching replaces the brushes and segmented commutators of traditional permanent magnet DC motors. Brushless motors may be of the variable reluctance, permanent magnet, or hybrid type. Variable reluctance brushless motors are characterized by having an iron core rotor follow or chase sequentially shifting magnetic fields of the stator coils to facilitate rotational motion of the rotor. Permanent magnet brushless motors are characterized by having the sequentially energized field coils attract or repel a permanent magnet rotor into rotational motion.
Electric motors are used to rotate loads in a variety of applications. One such application is in mass storage devices, such as hard disk drives. A hard disk drive generally includes a stack of rotating disks or platters and a spindle motor, which may be a salient pole brushless DC motor, for rotating the disks. The drive also includes one or more electromagnetic read/write heads which fly above the surface of the disks, an actuator motor (also known as a voice coil motor or VCM) which controls the positioning of the read/write heads, power circuitry to provide electrical power to the spindle and voice coil motors, circuitry for processing the data read from and/or written to the drive, and control circuitry to control the operation of the spindle and voice coil motors. The platters are typically rotated at a generally constant angular speed while the read/write heads read from or write to circular tracks on the platters. The mass storage device spindle motors are commonly multiple phase motors including a permanent magnet rotor and three electrical windings. The three electrical windings are related to the three phases of the motor. Three phase currents flow through the motor windings, typically at a 120 electrical degree phase relationship with respect to one another. The phase currents create a rotating electric field which causes angular rotation of the permanent magnet rotor.
The electromagnetic read/write heads read data from a disk platter by sensing flux changes on the magnetic surface of the platter as it passes beneath the read/write head. In order to synchronize the data being read from the disk with the operation of the data processing circuitry, it is necessary to carefully control the rotational speed of the disks. This is accomplished by controlling the current delivered to the spindle motor phase windings. The phase currents may be generated by the control circuitry in a variety of fashions. One method is to provide pulse-width-modulated (PWM) signals to the motor windings, wherein the timing of the individual PWM signals provided to each motor phase is determined by a control circuit. The duty cycle of the pulse width modulation signal therefore determines the average current delivered to the spindle motor. Another mode of current control is known as linear current control. The spindle motor control circuitry adjusts the level of current delivered by the power circuitry according to a desired motor performance parameter, such as speed and/or position.
Power is delivered to the motor phases through selectively energizing and de-energizing the individual phase windings. This process is known as commutation, and is accomplished via the control circuit. In order to rotate the disk drive motor in a given direction from startup and to maintain a desired rotational speed and torque at steady state, a commutation sequence or scheme is employed according to the present rotor position. This ensures that the proper phase windings are energized at appropriate times and polarities in order to provide the mutual attraction and/or repulsion between the phase windings and the rotor magnetic poles which results in the desired angular rotor motion.
To ensure proper rotational movement, it is essential to determine the position of the rotor with respect to the de-energized stator windings (or with respect to the energized windings). By knowing this position (sometimes referred to as commutation position), the stator windings can be energized in the appropriate sequence to create a revolving magnetic field in the motor to exert the desired rotational torque on the rotor. Rotor position has previously been detected by employing one or more transducers within the motor to sense the position of the rotor relative to the active stator windings.
However, the use of such transducers to determine commutation position has several drawbacks. First, these sensors increase production costs due to the need for sophisticated positional adjustment and increased wiring. Moreover, the space required for commutation position sensors is also a significant disadvantage in that valuable space is consumed within the motor housing. With an ever-increasing premium on space and cost efficiency, several attempts have been made to create sensorless commutation position feedback systems to replace the need for commutation position sensors within such motors.
The commutation control circuit is provided with rotor position information feedback (as well as rotor speed information), which is used to generate appropriate commutation signals for the motor windings. This position information may be obtained from some form of position sensor, or from measurements of back electromotive force (emf). Rotational position sensors include hall effect devices, magnetic sensors, optically encoded disks, resolvers, and other devices providing an indication of the relative positions of the rotor and stator to the control circuit via separate sensor signals. These devices, however, add to the cost and complexity of a motor, as well as occupying valuable physical space. Rotor position has also been heretofore determined or estimated based on a periodic measurement of back emf. However, the back emf in a motor winding varies as a function of motor current, and is thus an inaccurate measure of true rotor position. In addition, the back emf signal magnitude is proportional to the motor speed if the air gap flux is constant with respect to rotor position, and hence is difficult to measure at low motor speeds.
Conventional disk drive motors are commonly operated at approximately 5,200 rpm. However, the continuing trend in mass storage devices is toward higher data densities and faster data read and write access. As the data density (sometimes expressed in tracks per inch or TPI) increases in such devices, and as access times are reduced, higher motor speeds and improved speed control accuracy are desirable. For example, mass storage disk drive motor speeds of 7,200 have been introduced. Although conventional speed and position control methods are adequate for lower speeds and data densities, improvements are desirable in order to achieve higher rotational speeds as well as improved control accuracy for disk drive motors, without adding cost to the mass storage device. Accordingly, improved methods and systems for measuring rotor position are desirable to enable increased motor speed as well as improved motor control accuracy in high current, high speed, and/or high current change rate applications, which avoid or minimize the shortcomings of the back emf measurement techniques heretofore popular in motor control applications.
The present invention provides an improved sensorless apparatus and methodology for determining the rotor position in a polyphase electric motor which overcomes or minimizes the shortcomings of the prior art. The method includes measuring a mutual inductance associated with the motor, and determining rotor position according to the measured mutual inductance. For example, the inventors of the present invention have found that an equivalent mutual inductance of two energized phase windings in a three phase spindle motor passes through zero when a rotor pole or a rotor pole boundary is aligned with a third de-energized (e.g., floating) winding. Thus, the rotor position may be detected via correlating a detected zero crossing or polarity change in the mutual inductance with the position of a rotor pole.
In addition, it has also been found that the mutual inductance zero crossing does not vary with motor current or speed, as does the back emf signal zero crossing of such a floating phase winding. Thus, the employment of a mutual inductance zero crossing or polarity change measurement in determining the rotor position provides improved accuracy in both high current and/or low speed motor operation, over conventional back emf measurement methods and systems. This rotor position determination may be employed advantageously in motor commutation control circuits, such as are found in mass storage device spindle motor drive circuitry, although it will be appreciated that the invention finds utility in other applications as well.
According to one aspect of the present invention, there is provided a method of detecting rotor position in a polyphase electric motor, comprising measuring a mutual inductance in the motor and determining the rotor position according to the measured mutual inductance. Further in accordance with the invention, determining the rotor position according to the measured mutual inductance may comprise detecting a polarity change in the measured mutual inductance, and determining the rotor position according to the mutual inductance polarity change. The measurement of the mutual inductance may include measuring a voltage signal in a floating phase of the motor, removing low frequency components from the floating phase voltage signal, rectifying the filtered floating phase voltage signal, and filtering high frequency components out of the rectified phase voltage signal.
The floating phase voltage signal may include a back emf component, a mutual inductance component, and a high frequency excitation component. Filtering the low frequency components out of the floating phase voltage signal may remove the back emf component of the measured voltage signal, and filtering the high frequency components out of the rectified phase voltage signal may remove the excitation component of the measured voltage signal. The resulting signal may thus represent primarily the mutual inductance component, whereby detecting a polarity change in the measured mutual inductance may comprise detecting the minimum value of the resulting signal.
The method may further include correlating one of a rotor pole and a rotor pole boundary with the floating phase position according to the mutual inductance polarity change. In this way, an accurate rotor position determination can be made independent of the motor current, thereby enabling higher accuracy motor position and speed control. For example, the mutual inductance polarity change or zero crossing may be used to control a commutation circuit. Further in this regard, where the polyphase electric motor includes three phases, the voltage signal may be measured in a floating phase of the motor while the other two phase windings are energized. The measured mutual inductance in this case may be the equivalent mutual inductance of the energized motor phase windings with respect to the de-energized phase winding, which has been found to cross through zero when rotor poles and pole boundaries are aligned with the de-energized floating phase winding.
According to another aspect of the invention, an apparatus is provided for detecting rotor position in a polyphase electric motor. The apparatus may include a first circuit adapted to measure a mutual inductance in the motor, and a second circuit adapted to determine rotor position according to the measured mutual inductance. The second circuit may further be adapted to detect a polarity change in the measured mutual inductance and to determine the rotor position according to the mutual inductance polarity change. In addition, the first circuit may include a sensor adapted to measure a voltage signal in a floating phase of the motor, a high pass filter adapted to remove low frequency components from the floating phase voltage signal, a rectifier adapted to rectify the filtered floating phase voltage signal, and a low pass filter adapted to remove high frequency components from the rectified phase voltage signal. Accordingly, the second circuit may be further adapted to detect the minimum value of a signal from the low pass filter. The measured voltage signal may comprise a back emf component, a mutual inductance component, and an excitation component, wherein the high pass filter removes the back emf component of the measured voltage signal, and wherein the low pass filter removes the excitation component of the measured voltage signal.
In accordance with yet another aspect of the invention, there is provided a method of detecting rotor position in a polyphase electric motor, which includes measuring a back emf zero crossing in the motor, measuring a current in the motor, and determining the rotor position according to the measured back emf zero crossing and the measured current. The rotor position determination may further include correlating the measured back emf zero crossing with the measured current.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.